Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches.

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.

Exploring Spin Distribution and Electronic Properties in FeN4-Graphene Catalysts with Edge Terminations.

Understanding the spin distribution in FeN4-doped graphene nanoribbons with zigzag and armchair terminations is crucial for tuning the electronic properties of graphene-supported non-platinum catalysts. Since the spin-polarized carbon and iron electronic states may act together to change the electronic properties of the doped graphene, we provide in this work a systematic evaluation using a periodic density-functional theory-based method of the variation of spin-moment distribution and electronic properties with the position and orientation of the FeN4 defects, and the edge terminations of the graphene nanoribbons. Antiferromagnetic and ferromagnetic spin ordering of the zigzag edges were considered. We reveal that the electronic structures in both zigzag and armchair geometries are very sensitive to the location of FeN4 defects, changing from semi-conducting (in-plane defect location) to half-metallic (at-edge defect location). The introduction of FeN4 defects at edge positions cancels the known dependence of the magnetic and electronic proper-ties of undoped graphene nanoribbons on their edge geometries. The implications of the reported results for catalysis are also discussed in view of the presented electronic and magnetic properties.

Structural and Reactivity Effects of Secondary Metal Doping into Iron-Nitrogen-Carbon Catalysts for Oxygen Electroreduction.

While improved activity was recently reported for bimetallic iron-metal-nitrogen-carbon (FeMNC) catalysts for the oxygen reduction reaction (ORR) in acid medium, the nature of active sites and interactions between the two metals are poorly understood. Here, FeSnNC and FeCoNC catalysts were structurally and catalytically compared to their parent FeNC and SnNC catalysts. While CO cryo-chemisorption revealed a twice lower site density of M-Nx sites for FeSnNC and FeCoNC relative to FeNC and SnNC, the mass activity of both bimetallic catalysts is 50-100% higher than that of FeNC due to a larger turnover frequency in the bimetallic catalysts. Electron microscopy and X-ray absorption spectroscopy identified the coexistence of Fe-Nx and Sn-Nx or Co-Nx sites, while no evidence was found for binuclear Fe-M-Nx sites. 57Fe Mössbauer spectroscopy revealed that the bimetallic catalysts feature a higher D1/D2 ratio of the spectral signatures assigned to two distinct Fe-Nx sites, relative to the FeNC parent catalyst. Thus, the addition of the secondary metal favored the formation of D1 sites, associated with the higher turnover frequency.

Operando Spectroscopic Analysis of Axial Oxygen-Coordinated Single-Sn-Atom Sites for Electrochemical CO2 Reduction.

Sn-based materials have been demonstrated as promising catalysts for the selective electrochemical CO2 reduction reaction (CO2RR). However, the detailed structures of catalytic intermediates and the key surface species remain to be identified. In this work, a series of single-Sn-atom catalysts with well-defined structures is developed as model systems to explore their electrochemical reactivity toward CO2RR. The selectivity and activity of CO2 reduction to formic acid on Sn-single-atom sites are shown to be correlated with Sn(IV)-N4 moieties axially coordinated with oxygen (O-Sn-N4), reaching an optimal HCOOH Faradaic efficiency of 89.4% with a partial current density (jHCOOH) of 74.8 mA·cm-2 at -1.0 V vs reversible hydrogen electrode (RHE). Employing a combination of operando X-ray absorption spectroscopy, attenuated total reflectance surface-enhanced infrared absorption spectroscopy, Raman spectroscopy, and 119Sn Mössbauer spectroscopy, surface-bound bidentate tin carbonate species are captured during CO2RR. Moreover, the electronic and coordination structures of the single-Sn-atom species under reaction conditions are determined. Density functional theory (DFT) calculations further support the preferred formation of Sn-O-CO2 species over the O-Sn-N4 sites, which effectively modulates the adsorption configuration of the reactive intermediates and lowers the energy barrier for the hydrogenation of *OCHO species, as compared to the preferred formation of *COOH species over the Sn-N4 sites, thereby greatly facilitating CO2-to-HCOOH conversion.

Unraveling the Electronic Structure and Dynamics of the Atomically Dispersed Iron Sites in Electrochemical CO2 Reduction.

Single-atom catalysts with a well-defined metal center open unique opportunities for exploring the catalytically active site and reaction mechanism of chemical reactions. However, understanding of the electronic and structural dynamics of single-atom catalytic centers under reaction conditions is still limited due to the challenge of combining operando techniques that are sensitive to such sites and model single-atom systems. Herein, supported by state-of-the-art operando techniques, we provide an in-depth study of the dynamic structural and electronic evolution during the electrochemical CO2 reduction reaction (CO2RR) of a model catalyst comprising iron only as a high-spin (HS) Fe(III)N4 center in its resting state. Operando 57Fe Mössbauer and X-ray absorption spectroscopies clearly evidence the change from a HS Fe(III)N4 to a HS Fe(II)N4 center with decreasing potential, CO2- or Ar-saturation of the electrolyte, leading to different adsorbates and stability of the HS Fe(II)N4 center. With operando Raman spectroscopy and cyclic voltammetry, we identify that the phthalocyanine (Pc) ligand coordinating the iron cation center undergoes a redox process from Fe(II)Pc to Fe(II)Pc-. Altogether, the HS Fe(II)Pc- species is identified as the catalytic intermediate for CO2RR. Furthermore, theoretical calculations reveal that the electroreduction of the Pc ligand modifies the d-band center of the in situ generated HS Fe(II)Pc- species, resulting in an optimal binding strength to CO2 and thus boosting the catalytic performance of CO2RR. This work provides both experimental and theoretical evidence toward the electronic structural and dynamics of reactive sites in single-Fe-atom materials and shall guide the design of novel efficient catalysts for CO2RR.

Doped Graphene To Mimic the Bacterial NADH Oxidase for One-Step NAD+ Supplementation in Mammals.

Nicotinamide adenine dinucleotide (NAD) is a critical regulator of metabolic networks, and declining levels of its oxidized form, NAD+, are closely associated with numerous diseases. While supplementing cells with precursors needed for NAD+ synthesis has shown poor efficacy in combatting NAD+ decline, an alternative strategy is the development of synthetic materials that catalyze the oxidation of NADH into NAD+, thereby taking over the natural role of the NADH oxidase (NOX) present in bacteria. Herein, we discovered that metal-nitrogen-doped graphene (MNGR) materials can catalyze the oxidation of NADH into NAD+. Among MNGR materials with different transition metals, Fe-, Co-, and Cu-NGR displayed strong catalytic activity combined with >80% conversion of NADH into NAD+, similar specificity to NOX for abstracting hydrogen from the pyridine ring of nicotinamide, and higher selectivity than 51 other nanomaterials. The NOX-like activity of FeNGR functioned well in diverse cell lines. As a proof of concept of the in vivo application, we showed that FeNGR could specifically target the liver and remedy the metabolic flux anomaly in obesity mice with NAD+-deficient cells. Overall, our study provides a distinct insight for exploration of drug candidates by design of synthetic materials to mimic the functions of unique enzymes (e.g., NOX) in bacteria.

Chemical Kinetic Method for Active-Site Quantification in Fe-N-C Catalysts and Correlation with Molecular Probe and Spectroscopic Site-Counting Methods.

Mononuclear Fe ions ligated by nitrogen (FeNx) dispersed on nitrogen-doped carbon (Fe-N-C) serve as active centers for electrocatalytic O2 reduction and thermocatalytic aerobic oxidations. Despite their promise as replacements for precious metals in a variety of practical applications, such as fuel cells, the discovery of new Fe-N-C catalysts has relied primarily on empirical approaches. In this context, the development of quantitative structure-reactivity relationships and benchmarking of catalysts prepared by different synthetic routes and by different laboratories would be facilitated by the broader adoption of methods to quantify atomically dispersed FeNx active centers. In this study, we develop a kinetic probe reaction method that uses the aerobic oxidation of a model hydroquinone substrate to quantify the density of FeNx centers in Fe-N-C catalysts. The kinetic method is compared with low-temperature Mössbauer spectroscopy, CO pulse chemisorption, and electrochemical reductive stripping of NO derived from NO2- on a suite of Fe-N-C catalysts prepared by diverse routes and featuring either the exclusive presence of Fe as FeNx sites or the coexistence of aggregated Fe species in addition to FeNx. The FeNx site densities derived from the kinetic method correlate well with those obtained from CO pulse chemisorption and Mössbauer spectroscopy. The broad survey of Fe-N-C materials also reveals the presence of outliers and challenges associated with each site quantification approach. The kinetic method developed here does not require pretreatments that may alter active-site distributions or specialized equipment beyond reaction vessels and standard analytical instrumentation.

Oxygen Reduction Reaction in Alkaline Media Causes Iron Leaching from Fe-N-C Electrocatalysts.

The electrochemical activity of modern Fe-N-C electrocatalysts in alkaline media is on par with that of platinum. For successful application in fuel cells (FCs), however, also high durability and longevity must be demonstrated. Currently, a limited understanding of degradation pathways, especially under operando conditions, hinders the design and synthesis of simultaneously active and stable Fe-N-C electrocatalysts. In this work, using a gas diffusion electrode half-cell coupled with inductively coupled plasma mass spectrometry setup, Fe dissolution is studied under conditions close to those in FCs, that is, with a porous catalyst layer (CL) and at current densities up to -125 mA·cm-2. Varying the rate of the oxygen reduction reaction (ORR), we show a remarkable linear correlation between the Faradaic charge passed through the electrode and the amount of Fe dissolved from the electrode. This finding is rationalized assuming that oxygen reduction and Fe dissolution reactions are interlinked, likely through a common intermediate formed during the Fe redox transitions in Fe species involved in the ORR, such as FeNxCy and Fe3C@N-C. Moreover, such a linear correlation allows the application of a simple metric─S-number─to report the material's stability. Hence, in the current work, a powerful tool for a more applied stability screening of different electrocatalysts is introduced, which allows on the one hand fast performance investigations under more realistic conditions, and on the other hand a more advanced mechanistic understanding of Fe-N-C degradation in CLs.

Chemical vapour deposition of Fe-N-C oxygen reduction catalysts with full utilization of dense Fe-N4 sites.

Replacing scarce and expensive platinum (Pt) with metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells has largely been impeded by the low oxygen reduction reaction activity of M-N-C due to low active site density and site utilization. Herein, we overcome these limits by implementing chemical vapour deposition to synthesize Fe-N-C by flowing iron chloride vapour over a Zn-N-C substrate at 750 °C, leading to high-temperature trans-metalation of Zn-N4 sites into Fe-N4 sites. Characterization by multiple techniques shows that all Fe-N4 sites formed via this approach are gas-phase and electrochemically accessible. As a result, the Fe-N-C catalyst has an active site density of 1.92 × 1020 sites per gram with 100% site utilization. This catalyst delivers an unprecedented oxygen reduction reaction activity of 33 mA cm-2 at 0.90 V (iR-corrected; i, current; R, resistance) in a H2-O2 proton exchange membrane fuel cell at 1.0 bar and 80 °C.

P-block single-metal-site tin/nitrogen-doped carbon fuel cell cathode catalyst for oxygen reduction reaction.

This contribution reports the discovery and analysis of a p-block Sn-based catalyst for the electroreduction of molecular oxygen in acidic conditions at fuel cell cathodes; the catalyst is free of platinum-group metals and contains single-metal-atom actives sites coordinated by nitrogen. The prepared SnNC catalysts meet and exceed state-of-the-art FeNC catalysts in terms of intrinsic catalytic turn-over frequency and hydrogen-air fuel cell power density. The SnNC-NH3 catalysts displayed a 40-50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Additional benefits include a highly favourable selectivity for the four-electron reduction pathway and a Fenton-inactive character of Sn. A range of analytical techniques combined with density functional theory calculations indicate that stannic Sn(IV)Nx single-metal sites with moderate oxygen chemisorption properties and low pyridinic N coordination numbers act as catalytically active moieties. The superior proton-exchange membrane fuel cell performance of SnNC cathode catalysts under realistic, hydrogen-air fuel cell conditions, particularly after NH3 activation treatment, makes them a promising alternative to today's state-of-the-art Fe-based catalysts.

Narrow resection margins are not associated with mortality or recurrence in patients with Merkel cell carcinoma: A retrospective study.

BACKGROUND: Wide local excision constitutes the standard of care for Merkel cell carcinoma, but the optimal margin width remains controversial. OBJECTIVES: To assess whether narrow margins (0.5-1 cm) were associated with outcome. METHODS: Patients were recruited from a retrospective French multicentric cohort and included if they had had excision of primary tumor with minimum lateral margins of 0.5 cm. Factors associated with mortality and recurrence were assessed by multivariate regression. RESULTS: Among the 214 patients included, 58 (27.1%) had undergone excision with narrow margins (0.5-1 cm) versus 156 (72.9%) with wide margins (>1 cm). During a median follow-up of 50.7 months, cancer-specific survival did not differ between groups (5-year specific survival rate 76.8% [95% confidence interval 61.7%-91.9%] and 76.2% [95% confidence interval 68.8%-83.6%], respectively). Overall survival, any recurrence-free survival, and local recurrence-free survival did not significantly differ between groups. Cancer-specific mortality was associated with age, male sex, American Joint Committee on Cancer stage III, and presence of positive margins. LIMITATIONS: Retrospective design, heterogenous baseline characteristics between groups. CONCLUSION: Excision with narrow margins was not associated with outcome in this cohort, in which most patients had clear margins and postoperative radiation therapy. Residual tumor, mostly found on deep surgical margins, was independently associated with prognosis.

Potential-Induced Spin Changes in Fe/N/C Electrocatalysts Assessed by In Situ X-ray Emission Spectroscopy.

The commercial success of the electrochemical energy conversion technologies required for the decarbonization of the energy sector requires the replacement of the noble metal-based electrocatalysts currently used in (co-)electrolyzers and fuel cells with inexpensive, platinum-group metal-free analogs. Among these, Fe/N/C-type catalysts display promising performances for the reduction of O2 or CO2 , but their insufficient activity and stability jeopardize their implementation in such devices. To circumvent these issues, a better understanding of the local geometric and electronic structure of their catalytic active sites under reaction conditions is needed. Herein we shed light on the electronic structure of the molecular sites in two Fe/N/C catalysts by probing their average spin state with X-ray emission spectroscopy (XES). Chiefly, our in situ XES measurements reveal for the first time the existence of reversible, potential-induced spin state changes in these materials.

Engineering Fe-N Doped Graphene to Mimic Biological Functions of NADPH Oxidase in Cells.

NADPH oxidase (NOX) as a transmembrane enzyme complex controls the generation of superoxide that plays important roles in immune signaling pathway. NOX inactivation may elicit immunodeficiency and cause chronic granulomatous disease (CGD). Biocompatible synthetic materials with NOX-like activities would therefore be interesting as curative and/or preventive approaches in case of NOX deficiency. Herein, we synthesized a Fe-N doped graphene (FeNGR) nanomaterial that could mimic the activity of NOX by efficiently catalyzing the conversion of NADPH into NADP+ and triggering the generation of oxygen radicals. The resulting FeNGR nanozyme had similar cellular distribution to NOX and is able to mimic the enzyme function in NOX-deficient cells by catalyzing the generation of superoxide and retrieving the immune activity, evidenced by TNF-α, IL-1ß, and IL-6 production in response to Alum exposure. Overall, our study discovered a synthetic material (FeNGR) to mimic NOX and demonstrated its biological function in immune activation of NOX-deficient cells.

Evolution Pathway from Iron Compounds to Fe1(II)-N4 Sites through Gas-Phase Iron during Pyrolysis.

Pyrolysis is indispensable for synthesizing highly active Fe-N-C catalysts for the oxygen reduction reaction (ORR) in acid, but how Fe, N, and C precursors transform to ORR-active sites during pyrolysis remains unclear. This knowledge gap obscures the connections between the input precursors and the output products, clouding the pathway toward Fe-N-C catalyst improvement. Herein, we unravel the evolution pathway of precursors to ORR-active catalyst comprised exclusively of single-atom Fe1(II)-N4 sites via in-temperature X-ray absorption spectroscopy. The Fe precursor transforms to Fe oxides below 300 °C and then to tetrahedral Fe1(II)-O4 via a crystal-to-melt-like transformation below 600 °C. The Fe1(II)-O4 releases a single Fe atom that diffuses into the N-doped carbon defect forming Fe1(II)-N4 above 600 °C. This vapor-phase single Fe atom transport mechanism is verified by synthesizing Fe1(II)-N4 sites via "noncontact pyrolysis" wherein the Fe precursor is not in physical contact with the N and C precursors during pyrolysis.

On the Influence of Oxygen on the Degradation of Fe-N-C Catalysts.

Fe-N-C catalysts containing atomic FeNx sites are promising candidates as precious-metal-free catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. The durability of Fe-N-C catalysts in fuel cells has been extensively studied using accelerated stress tests (AST). Herein we reveal stronger degradation of the Fe-N-C structure and four-times higher ORR activity loss when performing load cycling AST in O2 - vs. Ar-saturated pH 1 electrolyte. Raman spectroscopy results show carbon corrosion after AST in O2 , even when cycling at low potentials, while no corrosion occurred after any load cycling AST in Ar. The load-cycling AST in O2 leads to loss of a significant fraction of FeNx sites, as shown by energy dispersive X-ray spectroscopy analyses, and to the formation of Fe oxides. The results support that the unexpected carbon corrosion occurring at such low potential in the presence of O2 is due to reactive oxygen species produced between H2 O2 and Fe sites via Fenton reactions.

Activity-Selectivity Trends in the Electrochemical Production of Hydrogen Peroxide over Single-Site Metal-Nitrogen-Carbon Catalysts.

Nitrogen-doped carbon materials featuring atomically dispersed metal cations (M-N-C) are an emerging family of materials with potential applications for electrocatalysis. The electrocatalytic activity of M-N-C materials toward four-electron oxygen reduction reaction (ORR) to H2O is a mainstream line of research for replacing platinum-group-metal-based catalysts at the cathode of fuel cells. However, fundamental and practical aspects of their electrocatalytic activity toward two-electron ORR to H2O2, a future green "dream" process for chemical industry, remain poorly understood. Here we combined computational and experimental efforts to uncover the trends in electrochemical H2O2 production over a series of M-N-C materials (M = Mn, Fe, Co, Ni, and Cu) exclusively comprising atomically dispersed M-Nx sites from molecular first-principles to bench-scale electrolyzers operating at industrial current density. We investigated the effect of the nature of a 3d metal within a series of M-N-C catalysts on the electrocatalytic activity/selectivity for ORR (H2O2 and H2O products) and H2O2 reduction reaction (H2O2RR). Co-N-C catalyst was uncovered with outstanding H2O2 productivity considering its high ORR activity, highest H2O2 selectivity, and lowest H2O2RR activity. The activity-selectivity trend over M-N-C materials was further analyzed by density functional theory, providing molecular-scale understandings of experimental volcano trends for four- and two-electron ORR. The predicted binding energy of HO* intermediate over Co-N-C catalyst is located near the top of the volcano accounting for favorable two-electron ORR. The industrial H2O2 productivity over Co-N-C catalyst was demonstrated in a microflow cell, exhibiting an unprecedented production rate of more than 4 mol peroxide gcatalyst-1 h-1 at a current density of 50 mA cm-2.

Electroreduction of CO2 on Single-Site Copper-Nitrogen-Doped Carbon Material: Selective Formation of Ethanol and Reversible Restructuration of the Metal Sites.

It is generally believed that CO2 electroreduction to multi-carbon products such as ethanol or ethylene may be catalyzed with significant yield only on metallic copper surfaces, implying large ensembles of copper atoms. Here, we report on an inexpensive Cu-N-C material prepared via a simple pyrolytic route that exclusively feature single copper atoms with a CuN4 coordination environment, atomically dispersed in a nitrogen-doped conductive carbon matrix. This material achieves aqueous CO2 electroreduction to ethanol at a Faradaic yield of 55 % under optimized conditions (electrolyte: 0.1 m CsHCO3 , potential: -1.2 V vs. RHE and gas-phase recycling set up), as well as CO electroreduction to C2 -products (ethanol and ethylene) with a Faradaic yield of 80 %. During electrolysis the isolated sites transiently convert into metallic copper nanoparticles, as shown by operando XAS analysis, which are likely to be the catalytically active species. Remarkably, this process is reversible and the initial material is recovered intact after electrolysis.

Structural Descriptors of Zeolitic-Imidazolate Frameworks Are Keys to the Activity of Fe-N-C Catalysts.

Active and inexpensive catalysts for oxygen reduction are crucially needed for the widespread development of polymer electrolyte fuel cells and metal-air batteries. While iron-nitrogen-carbon materials pyrolytically prepared from ZIF-8, a specific zeolitic imidazolate framework (ZIF) with sodalite topology, have shown enhanced activities toward oxygen reduction in acidic electrolyte, the rational design of sacrificial metal-organic frameworks toward this application has hitherto remained elusive. Here, we report for the first time that the oxygen reduction activity of Fe-N-C catalysts positively correlates with the cavity size and mass-specific pore volume in pristine ZIFs. The high activity of Fe-N-C materials prepared from ZIF-8 could be rationalized, and another ZIF structure leading to even higher activity was identified. In contrast, the ORR activity is mostly unaffected by the ligand chemistry in pristine ZIFs. These structure-property relationships will help identifying novel sacrificial ZIF or porous metal-organic frameworks leading to even more active Fe-N-C catalysts. The findings are of great interest for a broader application of the class of inexpensive metal-nitrogen-carbon catalysts that have shown promising activity also for the hydrogen evolution (Co-N-C) and carbon dioxide reduction (Fe-N-C and Mn-N-C).